Novel xerographic plate containing photoinjecting bis-benzimidazole pigments

ABSTRACT

An electrophotographic plate having a photoreceptor comprising a photoinjecting pigment selected from the class of bisbenzimidazole pigments and an active transport material which is substantially transparent in the wavelength region of xerographic use and capable of supporting charge carrier injection from the pigment. The photoinjecting bis-benzimidazole pigments have the property of being efficient both in photogeneration of charge carriers and subsequent injection of the charge carriers into hole and electron transport active transport materials. The photoinjecting pigment and active transport material system may be used in a binder or layer type photoreceptor. The structure may be imaged in the conventional xerographic mode which usually includes charging, exposure to light, and development.

United States Patent 1 Regensburger et a].

[ 51 Apr. 22, 1975 I 1 NOVEL XEROGRAPI-IIC PLATE CONTAININGPI-IOTOINJECTING BIS-BENZIMIDAZOLE PIGMENTS [75] Inventors: Paul J.Regensburger, Webster;

James J. Jakubowski, Rochester.

[21] Appl. No.: 348,398

Related US. Application Data [63] Continuation of'Scr. No. 94,067, Dec.1, 1970,

abandoned, which is a continuation-in-part of Ser. No. 14,467, Feb. 26.1970. abandoncd.

[52] [1.5. CI 96/].5; 96/].5 C; 96/16; 117/215; 117/218 [51] Int. Cl603g 5/00 [58] Field of Search 96/l.5, 1.5 C; 117/201, 117/215, 218

[56] References Cited UNITED STATES PATENTS 3,230,081 l/l966 Tomanek eta1 96/1 3,281,240 10/1966 Cassiers et a1. 96/1 3,287,123 11/1966 Hocgl96/1 5 3.331.687 7/1967 Kaschc 96/l,5

3,384,565 5/1968 Tulagiri et al 96/1.2

3,573,906 4/1971 Goffe 96/1.5 X 3,595,771 7/1971 Weigl 96/1 3,598,5828/1971 Herrick ct al 96/1.5 3,634,079 1/1972 Champ et a1. 96/1.53,704,121 11/1972 Makino et a1 96/1.5 X 3,713,820 1/1973 Champ ct a1.96/1.5

3.723.110 3/1973 Goffe 96/1.5 X 3,725,058 4/1973 Hayashi et a1.3.791.826 2/1974 Cherry et a1 96/1 .5

FOREIGN PATENTS OR APPLICATIONS 43-16198 7/1968 Japan 96/15 44-36742/1969 Japan 96/1.5

OTHER PUBLICATIONS Claus, Advances in Xerography: 1958-1962,"Photographic Science and Engineering, Vol. 7, No. 1, Jan-- Feb., 1963,pp. 5-14.

Primary E.\-aminerNorman G. Torchin Assistant Eraminer-John R. MillerAttorney, Agent, or Firm-James J. Ralabate; James P. OSullivan; DonaldM. MacKay 57 ABSTRACT An electrophotographic plate having aphotoreceptor comprising a photoinjecting pigment selected from theclass of bis-benzimidazole pigments and an active transport materialwhich is substantially transparent in the wavelength region ofxerographic use and capable of supporting charge carrier injection fromthe pigment. The photoinjecting bis-benzimidazole pigments have theproperty of being efficient both in photogeneration of charge carriersand subsequent injection of the charge carriers into hole and electrontransport active transport materials. The photoinjecting pigment andactive transport material system may be used in a binder or layer typephotoreceptor. The structure may be imaged in the conventionalxerographic mode which usually includes charging, exposure to light, anddevelopment.

22 Claims, 3 Drawing Figures PATENTEmPnzzi'rs 79,200

INVENTORS PAUL J. REGENSBURGER JAMES J. JAKUBOW Kl BY%M@Z ATTORN Y 1NOVEL XEROGRAPHIC PLATE CONTAINING PHOTOINJECTING BIS-BENZIMIDAZOLEPIGMENTS This is a continuation. of application Ser. No. 94.067. filedDec. 1. 1970. which in turn is a continuation-inpart of application Ser.No. 14.467. filed Feb. 26. l970 both now abandoned.

BACKGROUND OF THE INVENTION This invention relates in general toxerography and more specifically to a novel photosensitive device andmethod of use.

In the art of xerography. a xerographic plate containing aphotoconductive insulating layer is imaged by first uniformlyelectrostatically charging its surface. The plate is then exposed to apattern of activating electromagnetic radiation such as light. whichselectively dissipates the charge in the illuminated areas of thephotoconductive insulator while leaving behind a latent electrostaticimage in the non-illuminated areas. This latent electrostatic image maythen be developed to form a visible image by depositing finely dividedelectroscopic marking particles on the surface of the photoconductiveinsulating layer.

A photoconductive layer for use in xerography may be a homogeneous layerofa single material such as vitreous selenium or it may be a compositelayer containing a photoeonductor and another material. One type ofcomposite photoconductive layer used in xerography is illustrated byU.S. Pat. No. 3.12 l .006 to Middleton and Reynolds which describes anumber of binder layers comprising finely-divided particles of aphotoconductive inorganic compound dispersed in an electricallyinsulating organic resin binder. in its present commercial form. thebinder layer contains particles of zinc oxide uniformly dispersed in aresin binder and is coated on a paper backing.

In the particular examples of binder systems described in Middleton etal. the binder comprises a material which is incapable of transportinginjected charge Carriers generated by the photoeonductor particles forany significant distance. As a result. with the particular materialsdisclosed in the Middleton et al patent. the photoeonductor particlesmust be in substantially continuous particle-to-particle contactthroughout the layer in order to permit the charge dissipation requiredfor cyclic operation. With the uniform dispersion of photoeonductorparticles described in Middleton et al. therefore. a relatively highvolume concentration of photoeonductor. up to about 50 percent or moreby volume. is usually necessary in order to obtain sufficientphotoeonductor particle-to-particle contact for rapid discharge. It hasbeen found. however. that high photoeonductor loadings in the binderlayers of the resin type result in the physical continuity of the resinbeing destroyed. thereby significantly reducing the mechanicalproperties of the binder layer. Layers with high photoeonductor loadingsare often characterized by a brittle binder layer having little or noflexibility. On the other hand, when the photoeonductor concen trationis reduced appreciably below about 50 percent by volume. the dischargerate is reduced. making high speed cyclic or repeated imaging difficultor impossible.

U.S. Pat. No. 3,I2l.007 to Middleton et al teaches another type ofphotoeonductor which includes a two phase photoconductive binder layercomprising photoconductive insulating particles dispersed in ahomogeneous photoconductive insulating matrix. The photoconductor is inthe form of a particulate photoconductive inorganic crystalline pigmentbroadly disclosed as being present in an amount from about 5 to 80percent by weight. Photodischarge is said to be caused by thecombination of charge carriers generated in the photoconductiveinsulating matrix material and charge carriers injected from thephotoconductive crystalline pigment into the photoconductive insulatingmatrix.

U.S. Pat. No. 3.037.861 to Hoegl et al teaches that polyvinyl carbazoleexhibits some long-wave U. V. sensitivity and suggests that its spectralsensitivity be extended into the visible spectrum by the addition of dyesensitizers. Hoegl et al further suggests that other addi tives such aszinc oxide or titanium dioxide may also be used in conjunction withpolyvinyl carbazole. ln Hoegl et al. it is clear that the polyvinylcarbazole is intended to be used as a photoeonductor. with or withoutadditive materials which extend its spectral sensitivity.

In addition. certain specialized layer structures particularly designedfor reflex imaging have been proposed. For example. U.S. Pat. No.3.165.405 to Hoesterey utilizes a two layered zinc oxide binderstructure for reflex imaging. The Hoesterey patent utilizes two separatecontiguous photoconductive layers having different spectralsensitivities in order to carry out a particular reflex imagingsequence. The Hoesterey device utilizes the properties of multiplephotoconductive layers in order to obtain the combined advantages of theseparate photoresponse ofthe respective photoconductive layers.

It can be seen from a review of the conventional compositephotoconductive layers cited above. that upon exposure to light.photoconductivity in the layer structure is accomplished by chargetransport through the bulk of the photoconductive layer. as in the easeof vitreous selenium (and other homogeneous layer modifications). lndevices employing photoconductive binder structures. which includeinactive electrically insulating resins such as those described in theMiddleton et al. U.S. Pat. No. 3.121.006. conductivity or chargetransport is accomplished through high loadings of the photoconductivepigment allowing particle-to-particle contact of the photoconductiveparticles. In the case of photoconductive particles dispersed in aphotoconduc-- tive matrix. such as illustrated by the Middleton et al.U.S. Pat. No. 3,121,007. photoconductivity occurs through the generationof charge carriers in both the photoconductive matrix and thephotoeonductor pigment particles.

Although the above patents rely upon distinct mechanisms of dischargethroughout the photoconductive layer. they generally suffer from commondeficiencies in that the photoconductive surface during operation isexposed to the surrounding environment. and particularly in the case ofcycling xerography. susceptible to abrasion. chemical attack. heat. andmultiple exposures to light during cycling. These effects arecharacterized by a gradual deterioration in the electricalcharacteristics of the photoconductive layer resulting in the printingout of surface defects and scratches. localized areas of persistentconductivity which fail to retain an electrostatic charge. and high darkdischarge.

In addition to the problems noted above. these photoconductive layersrequire that the photoeonductor comprise either a hundred percent of thelayer. as in preferably contain a high proportion of photoconductivematerial in the binder configuration. The requirements of aphotoconductive layer containing all or a major proportion of aphotoconductive material further restricts the physical characteristicsof the final plate. drum or belt in that the physical characteristicssuch as flexibility and adhesion of the photoconductor to a supportingsubstrate are primarily dictated by the physical properties of thephotoconductor. and not by the resin or matrix material which ispreferably present in a minor amount.

Another form of composite photosensitive layer which has also beenconsidered by the prior art ineludes a layer of photoconductive materialwhich is covered with a relatively thick plastic layer and coated on asupporting substrate.

US. Pat. No. 3.04l.l66 to Bardeen describes such a configuration inwhich a transparent plastic material overlays a layer of vitreousselenium which is contained on a supporting substrate. The plasticmaterial is described as one having a long range for charge carriers ofthe desired polarity. In operation, the free surface of the transparentplastic is electrostatically charged to a given polarity. The device isthen exposed to activating radiation which generates a holeelectron pairin the photoconductive layer. The electron moves through the plasticlayer and neutralizes a positive charge on the free surface of theplastic layer thereby creating an electrostatic image. Bardeen, however,does not teach any specific plastic materials which will function inthis manner. and confines his examples to structures which use aphotoconductor material for the top layer.

French Pat. No. l,577.855 to Herrick et al describes a special purposecomposite photosensitive device adapted for reflex exposure by polarizedlight. One embodiment which employs a layer of dichroic organicphotoconductive particles arrayed in oriented fashion on a supportingsubstrate and a layer of polyvinyl carbazole formed over the orientedlayer of dichroic material. When charged and exposed to light polarizedperpendicularly to the orientation of the dichroic layer. the orienteddichroic layer and polyvinyl carbazole layer are both substantiallytransparent to the initial exposure light. When the polarized light hitsthe white background of the document being copied. the light isdepolarized, reflected back through the device and absorbed by thedichroic photoconductive material. In another embodiment. the dichroicphotoconductor is dispersed in oriented fashion throughout the layer ofpolyvinyl carbazole.

In view of the state of the art, it can readily be seen that there is aneed for a general purpose photoreceptor exhibiting acceptablephotoconductive characteristics and which additionally provides thecapability of exhibiting outstanding physical strength and flexibilityto be reused under rapid cyclic conditions without the progressivedeterioration of the xerographic properties due to wear. chemicalattack, and light fatigue.

OBJECTS OF THE INVENTION It is an object of this invention to provide anovel electrophotographic plate having a photoreceptor member containingphotoconductors which overcomes the above noted disadvantages.

Another object of the present invention is to provide a novelelectrophotographic imaging device having trophotographic memberphotosensitive pigments which are capable of highly efficient chargegeneration and injection.

Another object of this invention is to provide photoinjecting pigmentswhich are useful with either electron or hole active transportmaterials.

It is still another object of this invention to provide an operablyefficient photoreceptor portion of an elechaving relatively smallamounts of photosensitive material.

It is yet another object to provide a novel imaging system.

SUMMARY OF THE INVENTION The foregoing objects and others areaccomplished in accordance with this invention by providing anelectrophotographic plate having a photoreceptor member, comprisingactive transport material, which is capable of supporting photogeneratedcharge injection and transport, and a photoinjecting pigment which has ahigh efficiency of photogeneration of charge carriers and effectivecharge injection capability into said transport material. Thephotoinjecting pigments of the instant invention have maximumphotoresponse in the wavelength region in which most active transportmaterials are substantially transparent. In addition. thesephotoinjecting pigments are capable of injecting either photo-excitedelectrons or holes into the appropriate active transport materials withextraordinarily high efficiency under conditions of a practical appliedfield. The active transport material to be used in conjunction with thephotoinjecting pigments of the instant invention may be any materialcapable of supporting either hole or electron injection. provided it issubstantially non-absorbing in the particular wavelength region ofxerographic use which will coincide with the region in which thephotoconductor is photosensitive.

It should be understood that the active transport material does notfunction as a photoconductor in the wavelength region of use. As statedabove, holeelectron pairs are photogenerated in the photosensitivepigment and the electrons are then injected across a field modulatedbarrier into the active transport material and electron transport occursthrough said active transport material.

In accordance with the present invention it has been found that axerographic or electrophotographic sensi tive member can be preparedutilizing a photoinjecting pigment selected from the class ofbis-benzimidazole pigments in conjunction with electrostatically activetransport materials of either an electron or hole transport type.

It has now been found that bis-benzimidazole pigments, which are wellknown pigments. have both efficient photogeneration and injectioncharacteristics with electronically active transport materials. The mostuseful bis-benzimidazoles in such systems have been found to be thoserepresented by the following structures:

Trans-form R, and R each represent between one and four substituentstaken from the following classes: alkyl and substituted alkyl. aryl andsubstituted aryl. halogens, nitro and/or amino; in each case R and R maybe alike or different, and if several substituents are present on thesame ring they may be all identical or different. R and R may alsorepresent fused aromatic rings. e.g. the benzene may be replaced bynaphthalene. quinoline. or the like.

Bis-benzimidazole pigments suitable for use in the process of thisinvention may be synthesized by known methods, such as the generalreaction described by R. L. Shriner and R. W. Upson in Journal of theAmerican Chemical Society 64. 182 (1942). Typical of these known methodsin the preparation of the unsubstituted cis and trans-isomericstructures. given above, by reacting l.4.5,8-naphthalene tetracarboxylicacid with 0- phenylene diamine.

Of the compositions within the general class of bisbenzimidazoles thoseprepared by the reaction of l,4,5.8-naphthalene tetracarboxylic acidwith arylene diamines or heterocyclic diamines. and mixtures thereof arepreferred for use in the present active transport system since they aresimple and economically synthesized. are generally commerciallyavailable, have especially pure color and are highly photosensitive. Ofthese, the unsubstituted cis and trans forms of the bisbenzimidazolepigment structures. given above. which are commercially known asBordeaux RRN, the cis form (American Hoechst Company), and lndofastOrange, the trans form, (Allied Chemical Company) have produced optimumresults. Since the shade or tone of the compositions and the spectraland photosensitive responses vary slightly upon the substituent used,intermediate values of these variables may be obtained by mixing severaldifferent compositions. Any other bis-benzimidazole pigments, ormixtures thereof, may be used where suitable. Typical bis-benzimidazolepigments within the purview of the instant invention include thereaction products of 1.4.5.8-naphthalene tetracarboxylic acid withheterocyclic diamines. For use in an active transport photoreceptor. thebisbenzimidazoles may have other compositions added thereto tosensitize, enhance, synergize or otherwise modify its properties.

The bis-benzimidazole pigments of the instant invention are to bedistinguished fromother photosensitive .materials of the prior art inthat they are efficient in photogeneration and injection, and, inaddition, have excellent compatibility with most hole and electronactive transport materials. thereby enabling the use of a relatively lowapplied field in the corresponding xerographic photoreceptor member toeffect suitable injection and gain. The bis-benzimidazole pigments ofthe present invention also have optimum photosensitivity in thewavelength region of from about 4,000 to 6,500 Angstrom Units which isthe area of xerographic use over which an appropriate active transportmaterial must have a degree of transparency. By contrast, manyphotoconductors of the prior art. while beingphotosen- 'sitive in thiswavelength region. have not been'found to be sufficiently compatiblewith useful electronically active transport materials and are therebyinefficient with respect to injection of photogenerated charges into thesurrounding or adjacent active transport material. Therefore the use ofsaid photoconductive materials in combination with active transportmaterials requires an impractical applied field in excess of 5 X 10"volts/cm. Because of their unique properties the photoinjecting pigmentsof the instant invention can be used with transport materials inrelatively small quantities in either a layered or hinder structurexerographic photoreceptor.

A typical application of the instant invention consists of a supportingsubstrate such as an electrical conductor containing a photoconductorlayer overcoated with an active transport material. For example. thephotoconductor may comprise particles of lndofast Orange Toner. thetrans form of the unsubstituted bisbenzimidazole structure, given above,overcoated with a relatively thick layer of electron acceptor materialsuch as 2.4.7-trinitro-9-fluorenone (TNF). which is capable ofsupporting electron injection and transport. The distinctive nature ofthe pigment as well as its compatibility with the active transportmaterial enables the use of a relatively thin layer of thebis-benzimidazole without any loss of efficiency.

DESCRIPTION OF THE DRAWINGS Further objects of the invention, togetherwith additional features contributing thereto will be apparent from thefollowing description of one embodiment of the invention when read inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of another embodiment of anotherxerographic member contemplated by the instant invention. f

FIG. 2 is a schematic sectional view of another embodiment of anotherxerographic member contemplated by the instant invention. 7

FIG. 3 illustrates a discharge mechanism of injection by thephotoconductive pigments of the instant invention.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates one embodimentin the improved xerographic plate 10 according to this invention.Reference character ll designates a substrate or mechanical support. Thesubstrate may comprise a metal which is brass, aluminum. gold, platinum.steel or the like. It may be of any convenient thickness, rigid orflexible. in the form ofa sheet, web, cylinder. or the like. and may becoated with a thin blocking layer. It may also com prise such othermaterials as paper, metallized paper. plastic sheets covered with a thincoating of aluminum or copper oxide, or glass coated with a thin layerof chromium or tin oxide. It is usually preferred that the supportmember be somewhat electrically conductive or have a somewhat conductivesurface and that it be strong enough to permit a certain amount ofhandling. In certain instances, however. support 11 need not beconductive or may even be dispensed with entirely.

Reference character 12 designates a photoconductive single or unitarylayer which includes the photoinjecting bis-benzimidazole pigments ofthe instant invention. In particular the single layer comprises abisbenzimidazole selected from the group of bisbenzimidazoles preparedby reacting l,4,5.8- naphthalene tetracarboxylic acid with a suitablearylene diamine or heterocyclic diamine. Preferred bisbenzimidazolepigments are those prepared by the reaction of 1.4.5.8-naphthalenetetracarboxylic acid with -arylene diamines. All the aforementionedphotoinjecting bix-benzimidazoles have the property of being efficientphotogenerators and injectors into either hole or electron activetransport materials.

photoconductive single layer 12 of FIG. 1 may be of any suitablethickness used for carrying out its function in the xerographicinsulating member. thicknesses for this purpose range from 0.05 tomicrons. Thicknesses about 20 microns tend to produce undesirablepositive residual buildup in the pigment layer during the cycling andexcessive dark decay. while thicknesses below 0.05 microns becomeinefficient in absorbing impinging radiation. A range of from about 0.2to 5 microns is preferred since these thicknesses would insure maximumfunctionality of the photoconductor with a minimum amount of saidpigment substance and avoid the above mentioned problems with regard tothicknesses.

While reference character 12 of FIG. 1 designates a photoconductivesingle layer of photoinjecting pigment it is within the purview of theinstant invention that said layer may comprise photoinjecting pigmentdispersed in a matrix material. The matrix material may be any suitableorganic substance used for such purposes including inert matrix orbinder materials or one of the presently used active transport materialsdescribed. The concentration of the photoconductor material will varyaccording to which type of binder material is used and will range invalue from about 5 to 99 volume percent of the total photoconductivelayer. If an electronically inert binder material is used in combinationwith the photoinjecting pigments a volume ratio of at least percentphotoconductor to the electronically inert binder material is necessaryto effect particle-toparticle contact or proximity thereby renderinglayer 12 photoconductive throughout. The remarks with regard tothickness of the photoconductive single layer of FIG. 1 are applicablehere; namely, a range of from about 0.05 to 20 microns. with a range of0.2 to 5 microns being preferred. due to the variations of pigmentconcentration in the binder layer.

Because the photoreceptors of the instant invention will be exposed to awavelength region corresponding to the range of photoresponse of thepigment this will be the particular wavelength region to which theactive transport material must be substantially transparent. Asheretobefore mentioned the photoinjecting bisbenzimidazole pigmentsdescribed in the present invention have optimum photoresponse in thewavelength region of from about 4,000 to 6,500 Angstrom Units, the areaof xerographic use of the present pigmenttransport photoreceptor.Therefore exposure to a light source having this wavelength region ofemission enables the pigment to function at its maximum efficiency inabsorbing all impinging radiation and creating charge carriers.

Reference character 13 designates the active transport layer whichoverlies the photoinjecting pigment single layer 12. As pointed outabove. the active transport material can be either an electron or holetransport due to the distinctive nature and effectiveness of thephotoinjecting bis-benzimidazole pigments of the instant invention.Consistent with what has been said previously, the active transportmaterial to be used with the bis-benzimidazole pigments of the presentinvention must be substantially transparent in the wavelength region ofphotoresponse of the pigment which -region is the particular area ofxerographic use. The

bis-benzimidazole pigments of the present invention are photoresponsivein the wavelength region of from about 4,000 to 6,500 Angstroms. Theactive transport materials described are particularly useful withbisbenzimidazole pigments of the instant invention. These .include holetransport materials such as carbazole. N-

ethyl carbazole. N-isopropyl carbazole. N-phenylcarbazole.tetraphenylpyrene, l-methylpyrene. perylene. chrysene, anthracene.tetraphene, Z-phenyl naphthalene. azapyrene. fluorene, fluorenone,lethylpyrene, acetyl pyrene, 2,3-benzochrysene, 3,4- benzopyrene.1,4-bromopyrene, and phenyl-indole, polyvinyl carbazole. polyvinylpyrene. polyvinyl tetracene. polyvinyl perylene. and polyvinyltetraphene. Suitable electron transport materials include2.4.7-trinitro-9-fluorenone (TNF), 2,4,5,7-tetranitrofluorenone,dinitroanthracene, dinitroacridene. tetracyanopyrene, anddinitroanthraquinone.

It will be obvious to those skilled in the art that the use of anypolymer which contains the appropriate aromatic or heterocyclic chargechargge carrier transport such as carbazole. tetraphene, pyrene.2.4.7-trinitro-9- fluorenone. etc.. will function as an active transportmaterial. It is not the intent of the invention to restrict the type ofpolymer which can be employed as the transport material. Polyesters,polysiloxanes. polyamides. polyurethanes, and epoxies. as well as block.random or graft copolymers (containing the aromatic moiety) areexemplary of the various types of polymers which could be employed. Inaddition, electronically inactive polymers in which the active moiety isdispersed at high concentration can be employed.

The thickness of the active transport layer is not critical to thefunction of the xerographic member. However. the thickness of saidactive transport layer would be dictated by practical needs in terms ofthe amounts of electrostatic charge necessary to induce an applied fieldsuitable to effect electron injection and transport. Active transportlayer thicknesses of from about 5 to microns would be suitable, butthicknesses outside this range may be used. The ratio of the thicknessof the active transport layer to the photoconductor layer should bemaintained from about 2:1 to 200:].

The substantial or significant transparency of the active transportmaterial within the context of the instant invention..as exemplified byFIG. 1, means that a sufficient amount of radiation from a source mustpass through the active transport layer 13, in order that thephotoconductive layer 12, can function in its capacity as aphotogenerator and injector of charge carriers. More specifically,significant transparency is present in the wavelength region of fromabout 4,000 to 6.500 Angstrom Units impinges the pigment layer so as tocause discharge of a charged pigment-active transport photoreceptor.

It is not the intent of this invention to strictly restrict the choiceof active transport materials to those which are transparent in theentire visible region. For example. when the layered structure of FIG. 1is used with a transparent substrate. imagewise exposure may beaccomplished through the substrate without the light passing through thelayer of active transport material. In this case the active materialneed not be nonabsorbing in the wavelength region of use. Thisparticular application takes advantage of the injection properties ofthe present photoinjecting pigments and falls within the purview of theinstant invention. Other applications'where complete transparency is notrequired for the active material include the selective recording ofnarrow-band radiation such as that emitted from lasers, spectral patternrecognition. color coded form duplieation. and possible colorxerography.

While the active transport layer 13 of FIG. 1 may consist exclusively ofcharge transport material. for purposes of the present invention, thelayer may also comprise the charge transport material dispersed at asufficient concentration in a suitable inert binder material to effectparticle-to-particle contact or to effect sufficient proximity therebypermitting effective charge transport from the photoinjecting pigmentsof the instant invention through the layer. Generally there must be avolume ratio of at least 25 percent active transport material toelectronically inert binder material to obtain the desiredparticle-to-particle contact or proximity. Typical resin bindermaterials for the practice of the invention are polystyrene; siliconeresins such as DC-80l. DC-804. and DC-996 all manufactured by the DowCorning Corporation. Lexan. a polycarbonate, and SR-82 manufactured bythe General Electric Company; acrylic and methacrylic ester polymerssuch as Acryloid Al and Acryloid B72. polymerized ester derivatives ofacrylic and alpha-acrylic acids both supplied by Rohm and Haas Company.and Lucite 44. Lucite 45 and Lucite 46 polmerized butyl methacrylatessupplied by the E. l. du Pont de Nemours & Company: chlorinated rubbersuch as Parlon supplied by the Hercules Powder Company; vinyl polymersand copolymers such as polyvinyl chloride. polyvinyl acetate. etc.including Vinylite VYHH and VMCH manufactured by the BakeliteCorporation; cellulose esters and ethers such as ethyl cellulose.nitrocellulose. etc.; alkyd resins such as Glyptal 2469 manufactured bythe General Electric Company; etc. In addition. mixture of such resinswith each other or with plasticizers so as to improve adhesion.flexibility. blocking. etc. of the coatings may be used. Thus. Rezyl 869(a linseed oil-glycerol alkyd manufactured by American Cyanamid Company)may be added to chlorinated rubber to improve its adhesion andflexibility. Similarily. Vinylites VYHH and VMCH (polyvinylchloride-acetate copolymers manufactured by the Bakelite Company) may beblended together. Plasticizers include phthalates, phosphates. adipates.etc. such as tricresyl phosphate. dioctyl phthalate, etc. as is wellknown to those skilled in the plastics art.

Another embodiment of the instant invention is illustrated in FIG. 2.Here the photoreceptor layer consists of photoinjecting pigmentparticles 12 contained in an active transport matrix binder 13. Ingeneral. to attain the best combination of physical and electricalproperties. the pper limit for the photoconductive pigment or particlesmust be about 5 percent by volume of the active transport binder layer.A lower limit for the photoconductive particles of about 0.1 percent byvolume of the binder layer is required to insure that the lightabsorption coefficient is sufficient to give appreciable carriergeneration.

The thickness of the binder layer is not particularly critical. Layerthicknesses from about 2 to 100 microns have been found satisfactory.with a preferred thickness of about 5 to 50 yielding particularly goodresults.

The size of the photosensitive particles is not particularly critical inthe binder structure, but particles in a size range of about 0.01 to 1.0microns yield particularly satisfactory results.

While the layered configuration as described in FIG. 1 differsstructurally from the binder photoreceptor of F IG. 2, the functionalrelationship of the photosensitive material to the active transportmaterial is the same in that there is photogeneration in thephotosensitive particles and subsequent injection into the surroundingactive transport material. Therefore any description of the layeredconfiguration of FIG. 1, given above. relating to the nature of thematerials and the interactions with each other are applicable here withthe exception that. because of the proximity of the photosensitiveparticles. to the surface of the photoreceptor the binder plate ispreferably charged in the same polarity as the photogenerated chargeswhich can be transported by the active transport material. Therefore ifelectron transport material is being used as a binder then the plate ispreferably charged negatively while positive charging is preferred inthe case of hole transport material. In addition. the condition ofsubstantial transparency of the active transport material is necessaryhere to insure maximum functionality of the binder structure.

Another variation of the structures of F IGS. l and 2 consists of theuse of a blocking layer at the substratephotoreceptor interface. Such ablocking layer serves first to reduce potential leakage in the absenceof activating radiation. which leakage is known in the art as darkdecay. in addition. the blocking layer aids in sustainingan electricfield across the photoreceptor after the charging step. Any suitableblocking material may be used in thicknesses of from about 0.1 to 1micron. Typical materials include nylon. epoxy. aluminum oxide andinsulating resins of various types including polystyrene. butadienepolymers and copolymers. acrylic and methacrylic polymers. vinyl resins.alkyd resins. and cellulose base resin.

Reference character 13 in FIGS. 1 and 2 designate the active chargetransport material which acts as either an overlayer or a binder for thephotoinjecting pigment material 12. As heretobefore mentioned. thecharge transport material is capable of supporting charge injection fromthe pigment particles. or layer, and transporting said photogeneratedcharges under the influence of an applied field. In order to function inthe manner outlined above. the active transport material should besubstantially transparent, or nonabsorbing, to the particular wavelengthregion of pigment photosensitivity. With regard to the bisbenzimidazolepigments of the present invention the charge transport material shouldbe substantially nonabsorbing in the visible part of the electromagneticspectrum which ranges from about 4,000 to 6.500 Angstrom Units becausethe xerographically useful photoinjecting pigments have maximumphotoresponse to wavelengths in this region.

The active transport material which is employed in conjunction with thephotoconductive pigments in the instant invention is a material which isan insulator t0 the extent that an electrostatic charge placed on thecharge transport material is not conducted in the absence ofillumination at a rate to prevent the formation and retention of anelectrostatic latent image thereon. In general. this means that thespecific resistivity of the active transport material should be at least10' ohmscm. and preferably will be several orders higher. For optimumresults, however. it is preferred that the specific resistivity of theactive matrix material be such that overall resistivity of thephotoreceptor, in the absence of activating illumination or chargeinjection from the photoconductive pigments, be about ohms-cm.

In summary. it is clear that the photoinsulating portion of thexerographic members of the instant invention represented in FIGS. I and2 is divided into two functional components:

I. A photoinjecting pigment which photogenerates charge carriers uponexcitation by radiation within a particular wavelength region andinjects said photogenerated charge carriers into the adjacent activetransport material, and;

2. A substantially transparent active transport material which allowstransmission of radiation to the photoinjecting pigment. accepts thesubsequently photogenerated charge carriers from the photosensitivematerial, and actively transports these charge carriers to an oppositelycharged surface or substrate to effect neutralization.

This is more graphically illustrated by a simplified mechanism in FIG. 3where an electron-transport layered structure has been positivelycharged by means of corona charging. The activating radiationrepresented by the arrows I4 then passes through the transparent activetransport layer and impinges the pigment layer thereby creating ahole-electron pair. The electron and hole are then separated by theforce of the applied field and the electron injected across theinterface into' the active transport layer. There the photogeneratedelectron is transported by force of the electrostatic attraction throughthe active transport system to the surface where it neutralizes thepositive charge previously deposited by means of corona charging. Sinceonly photogenerated electrons can move in the presently illustratedelectron acceptor active transport layer, large changes in surfacepotential can result only when the electric field in the layeredstructure is such as to move the photogenerated electrons from thephotoconductor layer to the charged surface. It is necessary thereforethat in a layered configuration illustrated by FIG. 1 that an electrontransport material photoreceptor be charged positively and ahole-transport material photoreceptor be charged negatively. As pointedout above, the opposite is true when the system is a binder layer asillustrated in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS For purposes of affording thoseskilled in the art and a better understanding of the invention, thefollowing illustrative examples are given:

EXAMPLE I A plate or layered structure similar to that illustrated inFIG. I is prepared as follows:

I. A 0.2u nylon coated aluminum substrate is maintained at roomtemperature while a 0.8 micron thick layer of Indofast Orange Toner. thetrans form of the bis-benzimidazole structure, given above. manufacturedby the Allied Chemical Company, is vacuum evaporated thereon.

2. A 17 percent polymer stock solution is prepared by dissolving theappropriate amount of polyvinyl carbazole (Luvican Ml70 gradepoly-N-vinyl earbazole (PVK) from the BASF Chemical Company) in asolution of 180 grams of toluene and 20 grams of cyclohexanone.

EXAMPLE II An additional plate is made by the method of Example I usingBordeaux RRN, the trans form of the unsubstituted bis-benzimidazolestructure, given above, manufactured by the E. I. du Pont de Nemours Co.of Wilmington, Delaware. as the photoinjecting pigment.

The two plates made in Examples I and II were tested electrically by thefollowing technique: The sample is charged by.negative corona chargingto a potential of about 500 volts. The Since is then exposed to amonochromatic discharge light corresponding to a wavelength area inwhich each pigment has photoresponse. Sinc the photoinjecting pigmentsof the instant invention have maximum photoresponse, Amax, in thevisible region of the electromagnetic spectrum from about 4,000 to 6.500Angstrom Units the photoreceptors are exposed to a tungsten lampfiltered by an interference filter with a Angstrom Unit bad width,having its peak transmittance at about 4,500 Angstrom Units. Additionalmeasurements are taken with other filters having transmission peaksspaced about evenly through the entire region of from 4.000 to 6,500Angstroms. The initial voltage and resulting discharge. measured as(dV/dT) in each individual photodischarge experiment is monitored by aDC-type loop probe which is connected to a Keithley 610B electrometer tomeasure the voltages as a function of time resulting in a plot ofphotoresponse vs. field. From these experiments the maximum gain (G) andthe threshold field E,, that is. that field which gives rise to thelowest detectable discharge, are obtained. In addition, from the initialdischarge rate the gain (G) may be calculated.

The experimental methodology and the means of calculation are outlinedby P. Regensburger in Optical Sensitization of Charge Carrier Transportin PVK'." Photochemistry and Photobiology, 8, p. 429-40 (November,I968). Briefly, the gain is determined by plotting the initialxerographic gain (G) as a function of the applied field. The xerographicgain was calculated from the initial discharge rate where I is theincident photon flux, d the thickness of the layer, 6 the electricpermittivity, and e, the electronic charge. A xerographic gain of unitywould be observed if one charge carrier per incident photon were excitedand moved across the layer.

As can be seen from the results outlined in Table I the two platesexhibit a good xerographic maximum gain of greater than 35 percent. Alsoboth pigments require a relatively low threshold field of about 2.5volts/- micron which indicates that the photoinjecting pigments of theinstant invent. .m are capable of functioning under operating conditionsof most xerographic machinespln addition. the high discharge ratesconfirm what has been previously stated concerning the efficientphotogenerated charge injection properties of the bis'benzimidaxolepigments. The dissipation of the negatively charged PVK surfacegraphically illustrates the efficiency of hole injection into the activelayer.

TABLE" I G E dV/dT max t Neg Cris-Form As heretobefore mentioned. thephotoinjecting pigments of the present invention can be used withelectron transport active transport materials/[n Carrying outexperiments with an electron transport photoreceptor having'the instantphotoinjecting pigments the surface is positively charged andmeasurements conducted in the same manner outlined in Examples l and ll.It is found tht the electron transport photoreceptors have similarxerographic properties as the hole transport materials demonstrated inTable I; that is, there are acceptable xerographic gains and relativelylow threshold fields.

The present invention has been described with reference to certainspecific embodiments which have been presented in illustration of theinvention. It is to be understood however that numerous variations ofthe invention may be made and that it is intended to encompass suchvariation within the scope and spirit of the invention as described bythe following claims.

What is claimed is:

1. An electrophotographic plate having a photoreceptor member of fromabout 2 to 100 microns comprising a photoconductive material and anactive transport material both of which are compatible so as to supportefficient photogenerated charge injection from the photoconductivematerial, said photoconductive material being a photoinjecting pigmentbeing selected from the class of bis-benzimidazole pigments in an amountbetween about 0.1 and percent by volume of the active transport layerand said active transport material being a charge transport medium whichis substantially nonabsorbing in the wavelength region of from about4,000 to 6,500 Angstrom Units.

2. the electrophotographic plate of claim 1 in which the photoreceptormember comprises a bisbenzimidazole pigment dispersed in an activebinder transport material.

3. The electrophotographic plate of claim 1 in which the photoreceptormember is a layered configuration comprising a bis-benzimidazole pigmentsingle layer in contact with an active transport material overlayer. andwherein said active transport layer has a thickness of from about 5 to100 microns, said photoeonductor layer has a thickness of from 0.05 tomicrons and the ratio of the thickness of the active transport layer tothe photoconductor layer is from about 2:] to 200: l.

4. An electrophotographic plate having a photoreceptor member of fromabout 2 to 100 microns comprising a photoconductive material and anactive transport material both of which are compatible so as to supportefficient photogenerated charge injection, said photoconductive materialbeing a photoinjecting pigment being selected from the class ofbis-benzimidazole pigments prepared by the reaction of 1.4.5.8-naphthalene tetracarboxylic acid with arylene and heterocyclic diaminesin an amount between about 0.1 and -5 percent by volume of the activetransport layer and said active transport material being a chargetransport medium which is substantially non-absorbing in the wavelengthregion of from about 4,000 to 6,500 Angstroms.

5. The electrophotographic plate of claim 4 in which thebis-benzimidazole is the trans isomeric reaction product of1.4.5.8-naphthalene tetracarbonxylic acid with O-phenylene diamine.

6. The electrophotographic plate of claim 4 in which thebis-benzimidazole is the cis isomeric reaction product ofl,4,5.8-naphthalene tetracarboxylic acid with O-phenylene diamine.

7. The electrophotographic plate of claim 4 in which the photoreceptormember comprises the bisbenzimidazole pigment dispersed in an activetransport binder material.

8. The electrophotographic plate of claim 4 in which the photoreceptormember is a layered configuration comprising a bis-benzimidazole pigmentsingle layer and an active transport overlayer in contact with saidpigment layer. wherein the bis-benzimidazole pigment layer has athickness of from about 0.05 to 20 microns and the active transportoverlayer has a thickness of from about 5 to microns. and wherein theratio of the active transport material layer to the photoconductivelayer is from about 2:1 to 200:1.

9. The elctrophotographic plate of claim 4 in which the active transportmaterial is selected from the group of hole transport materialsconsisting of carbazole, N- ethyl carbazole, N-phenylcarbazole,tetraphenylpyrene, l-methylpyrene, perylene. chrysenc. fluorenc.fluorenone, anthracene, tetracene, tetraphene. l-phenyl-naphthalene.azapyrene, l-ethylpyrene, acetyl py rene, 2,3-benzochrysene,3,4-benzopyrene, 1.4- bromopyrene and phenyl indole, polyvinylcarbazole. polyvinyl pyrene, polyvinyl tetracene, polyvinyl perylene,and polyvinyl tetraphene.

10. The electrophotographic plate of claim 4 in which the activetransport material is selected from the group of electron transportsubstances consisting of 2,4,7-trinitro-9-fluorenone,2,4,5,7-tetranitrofluorenone, dinitroanthracene, dinitroacridene.tetracyanopyrene, dinitroanthraquinone, and polymeric materialscontaining said electron transport substance.

11. A method of imaging which comprises:

a. providing an electrophotographic plate having a photoreceptor memberof from about 2 to 100 microns comprising a photoconductive material andan active transport material. said photoconductive material being aphotoinjecting pigment being selected from the class ofbis-benzimidazole pigments in an amount between about 0.! and 5 percentby volume of the active transport layer. and said active transportmaterial being a charge transport medium which is substantiallynonabsorbing in the wavelength region of from about 4.000 to 6.500Angstroms.

b. uniformly charging said plate. and

c. exposing said plate to a source of radiation in the wavelength regionof from about 4.000 to 6.500 Angstroms whereby an electrostatic image isformed on the surface of said plate.

12. The method of claim 9 which further includes developing said latentimage to make it visible.

13. The method of claim 9 in which the substrate is substantiallytransparent and exposure is carried out through said substrate.

14. The plate of claim 1 wherein the transport material is polyvinylcarbazole.

15. The plate of claim 3 wherein the transport mate rial is polyvinylcarbazole.

16. The 'method of claim 11 wherein the transport material is polyvinylcarbazole.

17. The plate of claim 1 wherein the transport material is polyvinylcarbazole and the pigment is the cis form of bis-benzimidazodiazapyrenedione.

18. The plate of claim 1 wherein the transport mate-. rial is polyvinylcarbazole and the pigment is the' trans form of bis-benzimidazodiazapyrenedione.

19. The plate of claim 3 wherein the transport mate-i rial is polyvinylcarbazole and the pigment is the cis form of bis-benzimidazodiazapyrenedione.

20. The plate of claim 3 wherein the transport material is polyvinylcarbazole and the pigment is the trans form of bis-benzimidazodiazapyrenedione.

21. The method of claim 11 wherein the transport material is polyvinylcarbazole and the pigment is the cis form of bis-benzimidazodiazapyrenedione.

22. The method of claim 11 wherein the transport material is polyvinylcarbazole and the pigment is the trans form of bis-benzimidazodiazapyrenedione. i

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. I 3, 879, 200 DATED April 22,1975

tNVENTOR(S) Paul J, Regensburger et al It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

1. Column 3, line 26, change "holeelectron" to --ho1e-electron--,,

2, Column 7, line 3, change "bix-benzimidazoles to -bis-benzimidazoles-.

39 Column 7, line 8, change "thicknesses" to Thicknesses.

4. Column 8, line 22, change "charge chargge" to -moiety charge--, Q

5. Column 12, line 16, change "Since" to -sample 6, Column 12, line 19,change "Sinc" to -Since---- 7, Column 12, line 64, change"bis-benzimidaxole" to U bishenzimidazole O 8. Column 13, line 23,change "tht" to -that-o 9, Column 13, line 50, change "the" to--'Ihe---,,

. l0, Column 14, line 47, change "elctrophotographic" to-electrophotographic--,

ll. Column 14, line 52, change "l-phenyl-naphthalene" to--2phenyl-naphthalene-- Q 12,, Column 14, line 50, after"N-ethylcarbazole" put -N-iso'propyl carbazole-.

Signed and Scaled this second Day of December1975 [SML] RUTH c. msou rc.mansumu. mum A nesting Officer (ommissimwr uflmemx and Trademarks

1. AN ELECTROPHOTOGRAPHIC PLATE HAVING A PHOTORECEPTOR MEMBER OF FROMABOUT 2 TO 100 MICRONS COMPRISING A PHOTOCONDUCTIVE MATERIAL AND ANACTIVE TRANSPORT MATERIAL BOTH OF WHICH ARE COMPATIBLE SO AS TO SUPPORTEFFICIENT PHOTOGENERATED CHARGE INJECTION FROM THE PHOTOCONDUCTIVEMATERIAL, SAID PHOTOCONDUCTIVE MATERIAL BEING A PHOTOINJECTING PIGMENTBEING SELECTED FROM THE CLASS OF BIS-BENZIMIDAZOLE PIGMENTS IN AN AMOUNTBETWEEN ABOUT 0.1 AND 5 PERCENT BY VOLUME OF THE ACTIVE TRANSPORT LAYERAND SAID ACTIVE TRANSPORT MATERIAL BEING A CHARGE TRANSPORT MEDIUM WHICHIS SUBSTANTIALLY NONABSORBING IN THE WAVELENGTH REGION OF FROM ABOUT4,000 TO 6,500 ANGSTROM UNITS.
 1. An electrophotographic plate having aphotoreceptor member of from about 2 to 100 microns comprising aphotoconductive material and an active transport material both of whichare compatible so as to support efficient photogenerated chargeinjection from the photoconductive material, said photoconductivematerial being a photoinjecting pigment being selected from the class ofbis-benzimidazole pigments in an amount between about 0.1 and 5 percentby volume of the active transport layer and said active transportmaterial being a charge transport medium which is substantiallynonabsorbing in the wavelength region of from about 4,000 to 6,500Angstrom Units.
 2. the electrophotographic plate of claim 1 in which thephotoreceptor member comprises a bis-benzimidazole pigment dispersed inan active binder transport material.
 3. The electrophotographic plate ofclaim 1 in which the photoreceptor member is a layered configurationcomprising a bis-benzimidazole pigment single layer in contact with anactive transport material overlayer, and wherein said active transportlayer has a thickness of from about 5 to 100 microns, saidphotoconductor layer has a thickness of from 0.05 to 20 microns and theratio of the thickness of the active transport layer to thephotoconductor layer is from about 2:1 to 200:1.
 4. Anelectrophotographic plate having a photoreceptor member of from about 2to 100 microns comprising a photoconductive material and an activetransport material both of which are compatible so as to supportefficient photogenerated charge injection, said photoconductive materialbeing a photoinjecting pigment being selected from the class ofbis-benzimidazole pigments prepared by the reaction of1,4,5,8-naphthalene tetracarboxylic acid with arylene and heterocyclicdiamines in an amount between about 0.1 and 5 percent by volume of theactive transport layer and said active transport material being a chargetransport medium which is substantially non-absorbing in the wavelengthregion of from about 4,000 to 6,500 Angstroms.
 5. Theelectrophotographic plate of claim 4 in which the bis-benzimidazole isthe trans isomeric reaction product of 1,4,5,8-naphthalenetetracarbonxylic acid with 0-phenylene diamine.
 6. Theelectrophotographic plate of claim 4 in which the bis-benzimidazole isthe cis isomeric reaction product of 1,4,5,8-naphthalene tetracarboxylicacid with 0-phenylene diamine.
 7. The electrophotographic plate of claim4 in which the photoreceptor member comprises the bis-benzimidazolepigment dispersed in an active transport binder material.
 8. Theelectrophotographic plate of claim 4 in which the photoreceptor memberis a layered configuration comprising a bis-benzimidazole pigment singlelayer and an active transport overlayer in contact with said pigmentlayer, wherein the bis-benzimidazole pigment layer has a thickness offrom about 0.05 to 20 microns and the active transport overlayer has athickness of from about 5 to 100 microns, and wherein the ratio of theactive transport material layer to the photoconductive layer is fromabout 2:1 to 200:1.
 9. The elctrophotographic plate of claim 4 in whichthe active transport material is selected from the group of holetransport materials consisting of carbaZole, N-ethyl carbazole,N-phenylcarbazole, tetraphenylpyrene, 1-methylpyrene, perylene,chrysene, fluorene, fluorenone, anthracene, tetracene, tetraphene,1-phenyl-naphthalene, azapyrene, 1-ethylpyrene, acetyl pyrene,2,3-benzochrysene, 3,4-benzopyrene, 1,4-bromopyrene and phenyl indole,polyvinyl carbazole, polyvinyl pyrene, polyvinyl tetracene, polyvinylperylene, and polyvinyl tetraphene.
 10. The electrophotographic plate ofclaim 4 in which the active transport material is selected from thegroup of electron transport substances consisting of2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitrofluorenone,dinitroanthracene, dinitroacridene, tetracyanopyrene,dinitroanthraquinone, and polymeric materials containing said electrontransport substance.
 11. A method of imaging which comprises: a.providing an electrophotographic plate having a photoreceptor member offrom about 2 to 100 microns comprising a photoconductive material and anactive transport material, said photoconductive material being aphotoinjecting pigment being selected from the class ofbis-benzimidazole pigments in an amount between about 0.1 and 5 percentby volume of the active transport layer, and said active transportmaterial being a charge transport medium which is substantiallynonabsorbing in the wavelength region of from about 4,000 to 6,500Angstroms, b. uniformly charging said plate, and c. exposing said plateto a source of radiation in the wavelength region of from about 4,000 to6,500 Angstroms whereby an electrostatic image is formed on the surfaceof said plate.
 12. The method of claim 9 which further includesdeveloping said latent image to make it visible.
 13. The method of claim9 in which the substrate is substantially transparent and exposure iscarried out through said substrate.
 14. The plate of claim 1 wherein thetransport material is polyvinyl carbazole.
 15. The plate of claim 3wherein the transport material is polyvinyl carbazole.
 16. The method ofclaim 11 wherein the transport material is polyvinyl carbazole.
 17. Theplate of claim 1 wherein the transport material is polyvinyl carbazoleand the pigment is the cis form of bis-benzimidazodiaza pyrenedione. 18.The plate of claim 1 wherein the transport material is polyvinylcarbazole and the pigment is the trans form of bis-benzimidazodiazapyrenedione.
 19. The plate of claim 3 wherein the transport material ispolyvinyl carbazole and the pigment is the cis form ofbis-benzimidazodiaza pyrenedione.
 20. The plate of claim 3 wherein thetransport material is polyvinyl carbazole and the pigment is the transform of bis-benzimidazodiaza pyrenedione.
 21. The method of claim 11wherein the transport material is polyvinyl carbazole and the pigment isthe cis form of bis-benzimidazodiaza pyrenedione.